Underdamped passive ship stabilizer with deactivating means



UNDERDAMPED PASSIVE SHIP STABILIZER WI'TH DEACTIVATING MEANS Filed June 18, 1968 y 1970 s. B. FIELD E T A 2 Sheets-Sheet 1 F I6. I

FIG. 20

A H M W6 EN HM BA NV m8 LN mm SF FIG. 2b

July 21, 1970. 5'; ETAL 3,521,594

UNDERDAMPED PASSIVE -snn STABILIZER WITH :DEA-CTIVA'T-ING 'MEANS Filed June 18, 1968 V v 2 Sheets-Sheet 2 FIG 3 GYRO SENSING AND 20 CONTROL UNIT ,MOTOR l2 MOTOR K OR' OR SERVO 2 SERVO l I SEA/ WAVE UNSTABILIZEQ SHIP RO LL ROLL WITH I PRESENT SHIP ROLL WITH DAMPED TANK INVENTION B? C 7 7 2 7 DAMPED I7 V E TANK ROLL AMPLITUDE [2 z 4 2 I z z/j PRESET A D INVE NTION T l ME INVENTORS.

SHELDON YB. FIELD FIG. 4 FRANS v. A. PANGAL'ILA ORNEYS.

United States Patent 3,521,594 UNDERDAMPED PASSIVE SHIP STABILIZER WITH DEACTIVATING MEANS Sheldon B. Field, Floral Park, N.Y., and Frans V. A. Pangalila, Matawan Township, N.J., assignors to Flume Stabilization Systems, Inc., Hoboken, N.J., a corporation of New Jersey Continuation-impart of application Ser. No. 645,723, June 13, 1967. This application June 18, 1968, Ser. No. 744,272

Int. Cl. B63b 43/06 US. Cl. 114-125 6 Claims ABSTRACT OF THE DISCLOSURE An underdamped passive ship stabilizer with deactivating means operated during certain parts of the roll cycle to trap tank liquid at one end of the tank to lengthen the time during which the peak stabilizing moment is imparted to the ship. Since the tank is underdamped, a greater moment can be developed by reason of a greater liquid mass flowing per cycle of roll, and the tank liquid can be filled to a level significantly above the tuned level without creating phasing problems.

REFERENCE TO ANOTHER APPLICATION This is a continuation-in-part application of application Ser. No. 645,723, filed June 13, 1967 and now abandoned.

BACKGROUND With the advent of the principles disclosed in US. Pat. No. 3,054,373, passive ship stabilizers became for the first time practical and eventually generally accepted by the industry. The basic principles include a passive tank with a free surface liquid body therein which shifts from side to side in response to the ship roll. The tank liquid height is adjusted so that the tank oscillation is tuned to the ship roll. In order to avoid destabilization (increasing the roll magnitude) at frequencies away from the resonant frequency, the tank geometry is designed to significantly damp liquid passage which necessarily reduces the stabilizing moment near resonance. Notwithstanding the success enjoyed by these former stabilizers, the search continues to find ways of improving tank efiiciency in order to permit smaller tanks and less liquid volume without sacrificing stabilizing moment magnitude or increasing the chances of destabilization away from resonance.

The present invention improves on the stabilizers of the type described above by providing a tank geometry which houses the tank liquid in an underdamped condition so as to permit more liquid passage per cycle of roll than the abovedescribed stabilizers. Tank efiiciency is further enhanced by deactivating means which trap the liquid body at certain parts of the roll cycle and prevent liquid passage so as to extend the time during which a stabilizing moment is imparted to the vessel. At a predetermined time in the roll cycle, the deactivating means are opened to permit a rapid and substantially undamped liquid transfer to the other end of the tank so as to develop a properly phased moment during the next half roll cycle. The deactivating means are controlled by a gyro sensing and control unit which, if desired, may sense the ships roll and develop a signal representing the slope of the roll angle and comparing this signal value to predetermined standards the matching of which operates switches to control the deactivating means.

Since proper tank phasing is controlled by the deactivating means and not by the tank internal damping coefiicient, the tank liquid can be filled to heights greater than the tuned level to develop greater moments without creating phasing or destabilizing problems. This advantage 3,521,594 Patented July 21, 1970 ice DESCRIPTION OF THE DRAWINGS Other and further objects of the invention will become apparent with the following detailed description when taken in view of the appended drawings in which:

FIG. 1 is a diagrammatic illustration of a vertical transverse section through the hull of a vessel in which the present invention is arranged;

FIGS. 2a and 2b are horizontal sections of the tank stabilizer taken along line 22 of FIG. 1, and showing the deactivating means in the opened and closed positions;

FIG. 3 is a vertical transverse section taken along line 33 of FIG. 2; and

FIG. 4 is a series of wave curves illustrating the oscillating condition of the sea, ship unstabilized and stabilized, and tank liquid for a damped tank and the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS With reference to the drawings, the vessel generally indicated as 10 is fitted with a passive stabilizer 12 according to the present invention. Tank 12 preferably extends between two decks of the ship and extends from one side of the hull to the other. In the present example, the cross section of tank 12 is generally rectangular, and the fore and aft and athwartship dimensions may have any suitable values depending upon the distributed mass and particular roll characteristics of the vessel.

Tank 12 houses a liquid body filled to a level preferably at or above the level for which the natural oscillation of the tank liquid is tuned to the natural roll of the ship. With the height of stabilizer 12 arranged between decks of ship 10, the liquid body will be in a free surface condition throughout all parts of the roll cycle. The liquid medium can be of any suitable type, such as fresh water, seawater, bunker oil, fuel oil, and, if desired, special colloidal suspensions to increase the medium density.

With reference to FIGS. 20, 2b, and 3, deactivating means are provided for the tank which have the capability of sensing the ships roll and blocking liquid passage during certain parts of the roll cycle and releasing liquid for essentially undamped passage during other parts of the roll cycle. The deactivating means includes a plurality of upstanding plates or fins 14 secured for rotation to vertical axis formed by vertical rods 16 held in the top and bottom of the tank. The rods and fins are located at two regions symmetrical with and on opposite sides of the centerline of the tank and spaced from the tank ends to form wing tanks. Although fins 14 are arranged at only two locations in tank 12 to form two wing tanks and an interconnecting channel therebetween, it will be understood that the tank can be divided into as many compartments as necessary or desired by incorporating additional zones or stations of fins 14. The fins 14 and rods 16 are aligned in the fore and aft directions across the short dimension of the tank, and the fins are dimensioned so as to overlap generally as shown in FIG. 211 so as to close off or deactivate the local region of the tank. It should be understood that the closure structure may have any suitable design desired and the fin and rod arrangement as disclosed is merely one operable example.

Each set of fins and rods is operated by an electric or hydraulic motor 18 which, in turn, is controlled by a gyro sensing and control unit 20 which senses the roll of the ship and develops command signals accordingly.

The gyroscope used in unit 20 is a standard piece of marine equipment, and, preferably, the circuit receiving the gyro signals develops signals the value and sign of which may be related to the rate of change or slope of the roll angle. The output of unit 20 depends upon the electrical comparison of these values with predetermined set of values for controlling the positioning of fins 14. The circuit should also have the capability of developing a deactivating command whenever the gyro senses the ship roll at magnitude below a predetermined value. Thus, with low magnitude rolling, say, below 1 or 2, the stabilizer 12 will be deactivated.

The operation of the present invention will be best understood with reference to FIG. 4. It will be understood that the curves represent the ideal situation in which the sea, ship, and tank liquid oscillation all have the same period. Curve X represents the sea wave or upsetting wave imparted to the vessel. Curve Y represents the rolling action of an unstabilized vessel. Curve Z represents the stabilizing moment imparted to the vessel with a damped tank according to the principles of US. Pat. No. 3,054,373 and, superimposed thereon, the moment imparted to the vessel by the present invention.

During steady state operation, at time zero, the ship is passing through the horizontal and continues to roll toward the maximum at the quarter cycle. When the gyro senses the wave slope and sign at point a of curve Y, unit 20 operates motors 18 turning fins 14 to the positions shown in FIG. 2a. Thus, the tank liquid is released, and since there is very little damping, the tank liquid flows rapidly to the other end and accumulates there until unit 20 senses the wave slope at point b at which time fins 14 are closed to deactivate the tank and trap the liquid therein. In this way, the displaced liquid center of gravity is locked in off-outer position and continuously imparts a stabilizing moment to the ship during that part of the roll cycle from B to C (curve Z). When unit 20 senses the wave slope and sign at point of the roll curve, fins 14 are again opened and the tank liquid rapidly flows to the other end of the tank to impart a stabilizing moment in the opposite direction until wave slope D is sensed, at which time the tank liquid is again trapped by the closing of fins 14. This action continues as long as the ship rolls above a predetermined magnitude.

Two significant advantages of the present invention are immediately apparent from FIG. 4, curve Z. First, the stabilizing moment has a greater amplitude than the maximum moment developed by a damped tank having the same liquid mass therein. This result is achieved by virtue of the underdamped configuration of the present invention, permitting greater liquid movement per cycle of roll than in the case with the damped tank. Another significant advantage is that the maximum stabilizing moment is applied to the ship over a greater period of time, for example, from point B to point C, instead of the sinusoidal peak characteristic developed by the damped tank. The overall relative increase in tank efficiency for each half cycle of roll achieved by the present invention is represented by the area between the damped tank wave and the wave form of the present invention generally designated as E.

When ship rolls at a frequency removed from the resonant sea condition, gyro unit 20 can respond by developing a deactivating signal to deactivate the tank and thus avoid destabilization.

In the ideal situation, the wave slopes of curve Z for the present invention should be vertical; however, as a practical matter, the liquid mass cannot flow instantaneously along the tank, and consequently the invention operates with a generally linear and slightly inclined wave slope. In any event, the slope will be greater than the wave characteristics for the damped tank.

More particularly, the invention contemplates a method of operation of the stabilizing tank according to the following. The tank itself is characterized by as little damping as possible. Whereas it is inconceivable that one may obtain a damping coefficient of zero and thereby have a transfer of the mass of water within the tank which is undamped, it is possible to obtain a damping coetficient in actual practice which will be fairly low. The only way one can characterize the damping coefficient is by means of the phase relationship between the water in the tank and the ship. The criteria necessary for the designer is to have as little damping as possible and, in any event, so small that the phase lag between ship motion and water motion in the tank will be from about 20 to about 30. Smaller phase lags are not practically achievable since the tank, no matter how free of restrictions, will impose a certain amount of damping for normal liquid levels of up to six or seven feet.

The foregoing is in direct opposition to normal tank stabilization are which contemplates a phase lag of 90 in order to achieve the maximum stabilization moments. Damping of the tank system is preselected in these instances to provide damping coeflicients that will achieve the 90 phase relationship. As will be evident, the concept of the present invention proposes a tank system having a damping coefiicient totally different from and not suggested by normal stabilization systems in that the intention is to achieve a phase lag as low as possible, and, for practical purposes, this may be stated as from about 20' to about 30. The foregoing will serve as sufiicient characterization of the damping condition of the tank to enable one skilled in the art to effect the necessary design criteria.

With respect to the calibration and settings for the gyro sensing and control unit 20 to operate the fins 14-, the method of the present invention contemplates that the fins 14 will be maintained fully opened from about of the period prior to maximum amplitude to about of the period after the maximum amplitude of the roll angle, or as illustrated on FIG. 4, the curve identified by the legend Unstabilized Ship Roll, curve Y. Therefore, points A, a and points B, b of curves Z and Y, respectively, correspond to about of the period prior to maximum amplitude and about of the period after maximum amplitude of the roll angle as illustrated by the curve bearing the legend Unstabilized Ship Roll. Otherwise, the fins 14 are maintained completely closed.

Thus, in the practice of the method of the invention, the fins 14 are fully opened for approximately A: of the roll period correlated with roll angle as previously set forth, and thereafter fully closed for approximately /3 of the roll period correlated with roll angle as previously set forth, and successively opened for approximately /6 of the roll period and closed for approximately of the roll period with the method being repeated in the foregoing fashion as long as the tank system is in operation.

Various modifications can be made to the herein disclosed example of the present invention without depart ing from the spirit and scope thereof.

What is claimed is:

1. A passive tank stabilizer housing a body of liquid in a free-surface condition and having an internal tank geometry such that passage of liquid within the tank is substantially free of damping, the tank liquid having a height greater than the amount of tank liquid having a height greater than the amount of tank liquid necessary for the tank liquid oscillation to be tuned to the ship roll frequency, deactivating means comprising plate means divided the tank so as to form a wing tank at each end thereof and means for sensing the roll of the ship, said plate means being responsive to said sensing means to alternately open and close to trap the liquid selectively in said wing tanks to block the passage thereof at predetermined parts of the roll cycle and to release the tank liquid for passage during other parts of the roll cycle whereby maximum stabilizing moments will be achieved regardless of vsea state, the deactivating means deactivating the tank whenever the ship roll is below a predetermined magnitude, and means to simultaneously operate said plate means so that all of said plate means are either in their closed state or all of said plate means are in their open state.

2. A tank stabilizer as set forth in claim 1, wherein said deactivating means includes a gyro sensing and control unit for sensing the ships roll and developing deactivating and activating signals at predetermined points on the roll cycle, and closure means located within the tank for blocking liquid passage and permitting liquid passage in response to the signals from said unit.

3. A tank stabilizer as set forth in claim 1, wherein said deactivating means continuously deactivates the tank through a time from shortly before the rolling ship reaches the horizontal and to shortly after the ship has passed through the horizontal.

4. A tank stabilizer as set forth in claim 1, wherein the deactivating means permits continuous liquid passage through a part of the roll cycle including the time from shortly before the ship reaches the maximum roll magnitude to shortly after the ship passes through maximum roll magnitude.

5. A method of stabilizing a ship including a free surface tank type stabilizer having a pair of doors spaced from opposite ends of the tank, comprising the steps of sensing the rolling period of the ship, simultaneously opening both sets of doors for approximately /6 of the period from about of the period prior to maximum amplitude to about of the period after maximum amplitude in both senses, and simultaneously closing both sets of doors during the remainder of the period.

6. A method according to claim 5, wherein the tank system is characterized by a damping coefficient that will produce a phase lag of the oscillating mass of the tank of from about 20 to about 30.

References Cited UNITED STATES PATENTS 3,269,346 8/1966 Bell.

FOREIGN PATENTS 1,002,788 8/ 1965 Great Britain.

TRYGVE M. BLIX, Primary Examiner 

